Late at night, Heidi Becker positioned her subject in the crosshairs of an electron-shooting linear accelerator at the Curie Institute hospital in Paris.

By day, the accelerator was used to kill tumors in cancer patients. This time, its lethal gaze would be aimed at an early version of the star tracker aboard NASA’s Juno spacecraft, which is due to enter Jupiter’s orbit Monday.

The instrument will play a key role in helping scientists determine Juno’s orientation as it takes billions of high-definition measurements of Jupiter’s punishing magnetic field. Becker and her fellow engineers were mimicing the lethal forces the star tracker will face. They wanted to be sure that it would survive.

Jupiter “is the scariest place,” said Becker, who leads the mission’s radiation monitoring investigation at NASA’s Jet Propulsion Laboratory. “There isn’t anything like it anywhere else.”

Monday night, if all goes as planned, Juno will complete its looping, 1.8-billion-mile journey and slip into Jupiter’s embrace – the first spacecraft to orbit the gas giant since Galileo in 2003.

The rewards of the $1.1 billion mission are clear: to solve longstanding mysteries about the origins of our solar system, of Earth and, by extension, of life itself.

But as it explores the gas giant, Juno will have to fight for its own life. The craft will fly within 2,600 miles of the planet, closer than any previous satellite. As it completes 37 orbits over 20 months, Juno will have to withstand a brutal onslaught of radiation.

The high-energy electrons around Jupiter are like machine-gun fire: constant and extremely penetrating. After the initial volley, each ricocheting particle releases a spray of subatomic shrapnel that does even more damage.

“Jupiter is really, really hazardous,” said Scott Bolton, Juno’s principal investigator at the Southwest Research Institute. To withstand the barrage, most of Juno’s electronics are inside a titanium vault. “We’re an armored tank.”

Like its mythological counterpart, Jupiter is king of its realm: the largest and most powerful planet, more than twice as massive as all the others combined, and the first to be born from the swirling disk of gas and dust that once surrounded our nascent star.

It is also the most sun-like planet: made mostly of hydrogen and helium, with traces of heavier elements thrown in. Size aside, the main difference between Jupiter and the sun is that the planet has a larger share of those trace elements, including carbon, nitrogen and sulfur.

“We don’t know how Jupiter got enriched, but we know it’s very important,” Bolton said. “Because the stuff that Jupiter has more of is what we’re all made out of. It’s what the Earth is made out of. It’s what life comes from.”

Four centuries ago, Galileo Galilei’s observations of the planet and its four largest moons demonstrated that the Earth was not the center of the universe – a discovery that ultimately landed him under house arrest until his death. And yet, large as Jupiter looms in our minds, we know remarkably little about it.

Scientists don’t know if Jupiter’s Great Red Spot, that giant storm that could swallow the entire Earth, is merely an atmospheric phenomenon or a structure that penetrates far into the planet.

Nor do they know how much oxygen or water lies within the planet, a mystery that only deepened after NASA’s Galileo probe plunged into the gas giant in 1995 and found it to be surprisingly dry.

Juno will also be the first mission to orbit the planet’s poles, documenting its powerful northern and southern auroras, studying its colossal magnetic field up close and perhaps learning what exactly powers it.

Scientists also want to find out whether Jupiter has a hard center, a solid core of heavy metals such as iron and nickel, or whether it’s filled with hydrogen.